Biomedical electro-stimulators, such as cardiac pacemakers or neural stimulators, are often based on providing electric stimulation pulses to the human tissue with an electrode by discharge from a storage or holding capacitor in the electro-stimulator circuit. Between the stimulation discharges, this capacitor is charged from an energy source, e.g. a battery, to obtain the desired stimulation voltage for applying as a stimulation pulse to the tissue. Different strategies are used for charging the storage capacitor, thus affecting the energy efficiency of the electro-stimulator circuit. Energy efficiency is a crucial quality parameter especially for implanted battery-driven electro-stimulators such as pacemakers, since the implanted patient often needs a surgery in order to replace the battery. Lifetime of a given battery is directly influenced by the energy efficiency of the electro-stimulator circuit, which is in turn strongly determined by the amount of resistive losses.
An example of a charging circuit in a biomedical electro-stimulator can be found in U.S. Pat. No. 6,871,090. An inductive DC-DC converter (inductive boost converter), which can be modeled in steady state as a variable voltage source, charges a storage capacitor by converting energy from a battery. The charging capacitor is then charged to the voltage used for generating stimulation pulses. With such charging circuit it is possible to precisely control the charge voltage on the storage capacitor without the need for a power consuming linear voltage regulator.